When you are upon a bus and it's a pretty long journey and your thoughts start to wonder and the next thing you know, you're staring at the woman opposite's breasts because of many aspects - such as their firmness, their textures, their swollen appearance...
As you slowly come out of your mammory trance you realise with forbearing and unfortunate embarrassment, that the possessor of the breasts has been well aware of your intent gaze.

You have become a bus lesbian.

Hey, bus lesbian, stop staring at my baby feeding machines!

There was a bus lesbian on the way to college today hawking some sexy middle-aged momboob.